A k-page book embedding of a directed acyclic graph consists of a topological order of its vertices and a k-coloring of its edges, such that no two edges of the same color cross, that is, their endpoints do not alternate in the order. The minimum value of k for which such an embedding exists is referred to as the page number of the graph. In contrast to general directed acyclic planar graphs, which may have unbounded page number [SIAM J. Comput. 28(5), 1999], it was recently shown that directed acyclic outerplanar graphs have bounded page number. In particular, Jungeblut, Merker and Ueckerdt provided an upper bound of 24,776 on their page number [FOCS 2023: 1937-1952]. In this work, we focus on so-called monotone directed acyclic outerplanar graphs. Starting from a single edge, these graphs are constructed by iteratively connecting a new vertex to the endpoints of an existing edge on the outer face using either two incoming or two outgoing edges incident to it. These graphs have twist-number 4 [GD 2023: 135-151] (i.e., they admit a topological order in which no more than four edges pairwise cross), a property, which was leveraged by Jungeblut, Merker and Ueckerdt to show that their page number is at most 128. We lower this upper bound to 5 and we also provide a lower bound of 4. A notable consequence of our result is a significant improvement of the upper bound on the page number of general directed outerplanar graphs from 24,776 to 1,160.

Abstract A map is a partition of the sphere into interior-disjoint regions homeomorphic to closed disks. Some regions are labeled as nations, while the remaining ones are labeled as holes. A map in which at mostknations touch at the same point is ak-map, while it is hole-free if it contains no holes. A graph is a map graph if there is a bijection between its vertices and the nations of a map, such that two nations touch if and only the corresponding vertices are connected by an edge. We present a fixed-parameter tractable algorithm for recognizing map graphs parameterized by treewidth. Its time complexity is linear in the size of the graph. It reports a certificate in the form of a so-called witness, if the input is a yes-instance. Our algorithmic framework is general enough to test, for anyk, if the input graph admits ak-map or a hole-free k-map.

In this work, we continue the study of the area required for convex straight-line grid drawings of 3-connected plane graphs, which has been intensively investigated in the last decades. Motivated by applications, such as graph editors, we additionally require the obtained drawings to have bounded edge-vertex resolution, that is, the closest distance between a vertex and any non-incident edge in the drawing is lower bounded by a constant that does not depend on the size of the graph. We present a drawing algorithm that takes as input a 3-connected plane graph with n vertices and f internal faces, and computes a convex straight-line drawing with edge-vertex resolution at least 12 on an integer grid of size (n−2+a)×(n−2+a), where a=min⁡{n−3,f}. Our result improves the previously best-known area bound of (3n−7)×(3n−7)/2 by Chrobak, Goodrich and Tamassia.

Abstract We investigate the queue number of posets in terms of their width, that is, the maximum number of pairwise incomparable elements. A long-standing conjecture of Heath and Pemmaraju asserts that every poset of width w has queue number at most w. The conjecture has been confirmed for posets of width $$w=2$$ w = 2 via so-called lazy linear extension. We extend and thoroughly analyze lazy linear extensions for posets of width $$w > 2$$ w > 2 . Our analysis implies an upper bound of $$(w-1)^2 +1$$ ( w - 1 ) 2 + 1 on the queue number of width-w posets, which is tight for the strategy and yields an improvement over the previously best-known bound. Further, we provide an example of a poset that requires at least $$w+1$$ w + 1 queues in every linear extension, thereby disproving the conjecture for posets of width $$w > 2$$ w > 2 .

Abstract Bitonic st-orderings for st-planar graphs were introduced as a method to cope with several graph drawing problems. Notably, they have been used to obtain the best-known upper bound on the number of bends for upward planar polyline drawings with at most one bend per edge in polynomial area. For an st-planar graph that does not admit a bitonic st-ordering, one may split certain edges such that for the resulting graph such an ordering exists. Since each split is interpreted as a bend, one is usually interested in splitting as few edges as possible. While this optimization problem admits a linear-time algorithm in the fixed embedding setting, it remains open in the variable embedding setting. We close this gap in the literature by providing a linear-time algorithm that optimizes over all embeddings of the input st-planar graph. The best-known lower bound on the number of required splits of an st-planar graph with n vertices is $$n-3$$ n - 3 . However, it is possible to compute a bitonic st-ordering without any split for the st-planar graph obtained by reversing the orientation of all edges. In terms of upward planar polyline drawings in polynomial area, the former translates into $$n-3$$ n - 3 bends, while the latter into no bends. We show that this idea cannot always be exploited by describing an st-planar graph that needs at least $$n-5$$ n - 5 splits in both orientations. We provide analogous bounds for graphs with small degree. Finally, we further investigate the relationship between splits in bitonic st-orderings and bends in upward planar polyline drawings with polynomial area, by providing bounds on the number of bends in such drawings.

Abstract A k -queue layout is a special type of a linear layout, in which the linear order avoids $$(k+1)$$ ( k + 1 ) -rainbows, that is, $$k+1$$ k + 1 independent edges that pairwise form a nested pair. The optimization goal is to determine the queue number of a graph, which is defined as the minimum value of k for which a k -queue layout is feasible. Recently, Dujmović et al. [J. ACM, 67(4), 22:1–38, 2020] showed that the queue number of planar graphs is at most 49, thus settling in the positive a long-standing conjecture by Heath, Leighton and Rosenberg. To achieve this breakthrough result, their approach involves three different techniques: (1) an algorithm to obtain 2-queue layouts of outerplanar graphs, (2) an algorithm to obtain 5-queue layouts of planar 3-trees, and (3) a decomposition of a planar graph into so-called tripods. In this work, we push further each of these techniques to obtain the first non-trivial improvement of the upper bound on the queue number of planar graphs from 49 to $$42 $$ 42 .

The page-number of a directed acyclic graph (a DAG, for short) is the minimum k for which the DAG has a topological order and a k-coloring of its edges such that no two edges of the same color cross, i.e., have alternating endpoints along the topological order. In 1999, Heath and Pemmaraju conjectured that the recognition of DAGs with page-number 2 is NP-complete and proved that recognizing DAGs with page-number 6 is NP-complete (Heath and Pemmaraju (1999) [15]). Binucci et al. recently strengthened this result by proving that recognizing DAGs with page-number k is NP-complete, for every k≥3 (Binucci et al. (2019) [6]). In this paper, we finally resolve Heath and Pemmaraju's conjecture in the affirmative. In particular, our NP-completeness result holds even for st-planar graphs and planar posets.

An embedding of a graph in a book, called book embedding, consists of a linear ordering of its vertices along the spine of the book and an assignment of its edges to the pages of the book, so that no two edges on the same page cross. The book thickness of a graph is the minimum number of pages over all its book embeddings. For planar graphs, a fundamental result is due to Yannakakis, who proposed an algorithm to compute embeddings of planar graphs in books with four pages. Our main contribution is a technique that generalizes this result to a much wider family of nonplanar graphs, which is characterized by a biconnected skeleton of crossing-free edges whose faces have bounded degree. Notably, this family includes all 1-planar and all optimal 2-planar graphs as subgraphs. We prove that this family of graphs has bounded book thickness, and as a corollary, we obtain the first constant upper bound for the book thickness of optimal 2-planar graphs.

Abstract A queue layout of a graph G consists of a linear order of the vertices of G and a partition of the edges of G into queues , so that no two independent edges of the same queue are nested. The queue number of graph G is defined as the minimum number of queues required by any queue layout of G . In this paper, we continue the study of the queue number of planar 3-trees, which form a well-studied subclass of planar graphs. Prior to this work, it was known that the queue number of planar 3-trees is at most seven. In this work, we improve this upper bound to five. We also show that there exist planar 3-trees whose queue number is at least four. Notably, this is the first example of a planar graph with queue number greater than three.

A queue layout of a graph consists of a linear order of its vertices and a partition of its edges into queues, so that no two independent edges of the same queue are nested. The queue number of a graph is the minimum number of queues required by any of its queue layouts. A long-standing conjecture by Heath, Leighton and Rosenberg states that the queue number of planar graphs is bounded.This conjecture has been partially settled in the positive for several sub- families of planar graphs (most of which have bounded treewidth).

A queue layout of a graph consists of a linear order of its vertices and a partition of its edges into queues, so that no two independent edges of the same queue are nested. The queue number of a graph is the minimum number of queues required by any of its queue layouts. A long-standing conjecture by Heath, Leighton and Rosenberg [ SIAM J. Discrete Math., 5 (1992), pp. 398--412] states that the queue number of planar graphs is bounded. This conjecture has been partially settled in the positive for several subfamilies of planar graphs (most of which have bounded treewidth). In this paper, we make a further important step towards settling this conjecture. We prove that planar graphs of bounded degree (which may have unbounded treewidth) have bounded queue number. A notable implication of this result is that every planar graph of bounded degree admits a three-dimensional straight-line grid drawing in linear volume. Further implications are that every planar graph of bounded degree has bounded track number, and that every $k$-planar graph (i.e., every graph that can be drawn in the plane with at most $k$ crossings per edge) of bounded degree has bounded queue number.

Back in the eighties, Heath showed that every 3-planar graph is subhamiltonian and asked whether this result can be extended to a class of graphs of degree greater than three. In this paper we affirmatively answer this question for the class of 4-planar graphs. Our contribution consists of two algorithms: The first one is limited to triconnected graphs, but runs in linear time and uses existing methods for computing hamiltonian cycles in planar graphs. The second one, which solves the general case of the problem, is a quadratic-time algorithm based on the book embedding viewpoint of the problem.